linux-kernel - Re: [PATCH v2 19/49] KVM: x86: Add a macro to init CPUID features that ignore host kernel support

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Message-ID: <3e17a33c59c6a57a836c92c1bddc9bd6d36cafca.camel@redhat.com>
Date: Wed, 24 Jul 2024 13:40:51 -0400
From: Maxim Levitsky <mlevitsk@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Vitaly Kuznetsov
 <vkuznets@...hat.com>,  kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Hou Wenlong <houwenlong.hwl@...group.com>, Kechen Lu <kechenl@...dia.com>,
 Oliver Upton <oliver.upton@...ux.dev>, Binbin Wu
 <binbin.wu@...ux.intel.com>, Yang Weijiang <weijiang.yang@...el.com>,
 Robert Hoo <robert.hoo.linux@...il.com>
Subject: Re: [PATCH v2 19/49] KVM: x86: Add a macro to init CPUID features
 that ignore host kernel support

On Mon, 2024-07-08 at 15:36 -0700, Sean Christopherson wrote:
> On Thu, Jul 04, 2024, Maxim Levitsky wrote:
> > On Fri, 2024-05-17 at 10:38 -0700, Sean Christopherson wrote:
> > > +/*
> > > + * Raw Feature - For features that KVM supports based purely on raw host CPUID,
> > > + * i.e. that KVM virtualizes even if the host kernel doesn't use the feature.
> > > + * Simply force set the feature in KVM's capabilities, raw CPUID support will
> > > + * be factored in by kvm_cpu_cap_mask().
> > > + */
> > > +#define RAW_F(name)						\
> > > +({								\
> > > +	kvm_cpu_cap_set(X86_FEATURE_##name);			\
> > > +	F(name);						\
> > > +})
> > > +
> > >  /*
> > >   * Magic value used by KVM when querying userspace-provided CPUID entries and
> > >   * doesn't care about the CPIUD index because the index of the function in
> > > @@ -682,15 +694,12 @@ void kvm_set_cpu_caps(void)
> > >  		F(AVX512VL));
> > >  
> > >  	kvm_cpu_cap_mask(CPUID_7_ECX,
> > > -		F(AVX512VBMI) | F(LA57) | F(PKU) | 0 /*OSPKE*/ | F(RDPID) |
> > > +		F(AVX512VBMI) | RAW_F(LA57) | F(PKU) | 0 /*OSPKE*/ | F(RDPID) |
> > >  		F(AVX512_VPOPCNTDQ) | F(UMIP) | F(AVX512_VBMI2) | F(GFNI) |
> > >  		F(VAES) | F(VPCLMULQDQ) | F(AVX512_VNNI) | F(AVX512_BITALG) |
> > >  		F(CLDEMOTE) | F(MOVDIRI) | F(MOVDIR64B) | 0 /*WAITPKG*/ |
> > >  		F(SGX_LC) | F(BUS_LOCK_DETECT)
> > >  	);
> > > -	/* Set LA57 based on hardware capability. */
> > > -	if (cpuid_ecx(7) & F(LA57))
> > > -		kvm_cpu_cap_set(X86_FEATURE_LA57);
> > >  
> > >  	/*
> > >  	 * PKU not yet implemented for shadow paging and requires OSPKE
> > 
> > Putting a function call into a macro which evaluates into a bitmask is somewhat misleading,
> > but let it be...
> 
> And weird.  Rather than abuse kvm_cpu_cap_set(), what about adding another variable
> scoped to kvm_cpu_cap_init()?
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 0e64a6332052..b8bc8713a0ec 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -87,12 +87,10 @@ u32 xstate_required_size(u64 xstate_bv, bool compacted)
>  /*
>   * Raw Feature - For features that KVM supports based purely on raw host CPUID,
>   * i.e. that KVM virtualizes even if the host kernel doesn't use the feature.
> - * Simply force set the feature in KVM's capabilities, raw CPUID support will
> - * be factored in by __kvm_cpu_cap_mask().
>   */
>  #define RAW_F(name)                                            \
>  ({                                                             \
> -       kvm_cpu_cap_set(X86_FEATURE_##name);                    \
> +       kvm_cpu_cap_passthrough |= F(name);                     \
>         F(name);                                                \
>  })
>  
> @@ -737,6 +735,7 @@ do {                                                                        \
>  do {                                                                   \
>         const struct cpuid_reg cpuid = x86_feature_cpuid(leaf * 32);    \
>         const u32 __maybe_unused kvm_cpu_cap_init_in_progress = leaf;   \
> +       u32 kvm_cpu_cap_passthrough = 0;                                \
>         u32 kvm_cpu_cap_emulated = 0;                                   \
>         u32 kvm_cpu_cap_synthesized = 0;                                \
>                                                                         \
> @@ -745,6 +744,7 @@ do {                                                                        \
>         else                                                            \
>                 kvm_cpu_caps[leaf] = (mask);                            \
>                                                                         \
> +       kvm_cpu_caps[leaf] |= kvm_cpu_cap_passthrough;                  \
>         kvm_cpu_caps[leaf] &= (raw_cpuid_get(cpuid) |                   \
>                                kvm_cpu_cap_synthesized);                \
>         kvm_cpu_caps[leaf] |= kvm_cpu_cap_emulated;                     \
> 

I agree, this is better.

Best regards,
	Maxim Levitsky